1. Field of the Invention
The present invention relates to a fixing device, and to an image forming apparatus having the fixing device.
2. Description of the Related Art
In an image formation using an electrophotographic system, a photoreceptor charged to a uniform electric potential is exposed to light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred onto recording paper or the like, and the transferred developer on the recording paper is made to be fixed thereon so as to form a solid recording image.
One of significant development objectives is to increase an output speed in a set of steps in the image formation, that is, to increase the number of recording paper sheets which can be subjected to the image formation steps, for one minute. In an image forming apparatus of conventional design, of which output speed is 30 to 40 sheets/min., a process speed is 200 to 400 mm/sec. In development of recent years is provided an image forming apparatus of which output speed is 100 sheets/min. or more (conversion to A4 widthwise conveyance). With realization of such an enhancement of the output speed, an image forming apparatus of which process speed reaches 450 mm/sec. or more and furthermore, 600 mm/sec. has appeared.
For increase in the output speed, that is, increase in the process speed of image information in the image forming apparatus, a print (image forming) process can be adapted by increasing a charging potential, increasing an applying voltage to a developing device, enlarging a developer container, or other changes.
However, in the fixing device of the invention, a heat quantity which can be given to fuse unfixed toner on recording paper, in other words, a fixing temperature which rises by heating a fixing roller is the same as that in the conventional fixing device, regardless of increase in the process speed. This is because setting the fixing temperature to a too high degree leads such a harmful effect that the toner is excessively fused at all times and hot offset is constantly generated.
Accordingly, even when a set fixing temperature of the fixing roller is the same as that in the conventional fixing device, with an increase in the process speed, a speed of the recording paper passing a nip region formed by a heating roller and a pressure roller which constitute the fixing roller becomes extremely high, thereby generating a phenomenon that heat transfer of the heating roller runs short. Moreover, when the recording paper is passing through the nip region, the recording paper draws heat from the fixing roller, with the result that the heat is more frequently removed per unit time by the recording paper.
When the heat is removed from the fixing roller by the recording paper, a control unit conducts a control of increasing an inputting electricity to the heating roller so that the fixing roller recovers the set fixing temperature. With a higher frequency of the decrease in temperature due to heat removal and the increase in the inputting electricity owing to the control, a ripple (variation of temperature in practice around the set fixing temperature) becomes larger so that a frequency of overshooting the set fixing temperature becomes high and moreover, a range of overshot temperature becomes wide.
Such an increase in the process speed makes the fixing temperature more easily overshoot the set value, and when overshooting the set value, the toner is excessively fused. The excessive fusion of the toner leads decrease in mold-releasing property of a to-be-conveyed recording paper from the heating roller, and contamination of the heating roller.
In the meantime, the fixing device is provided with a peeling claw for peeling off the recording paper from the fixing roller. Assuming that the toner is excessively fused as described above, when the peeling claw peels off the recording paper from the fixing roller, the excessively fused toner on the recording paper may be adhered to the peeling claw, resulting in contaminating the peeling claw. Further, the toner accumulated on the peeling claw through continuation of a fixing operation may cause jam which indicates that recording paper being conveyed is clogged, contaminating the fixing roller, contaminating images lead by the contamination of the fixing roller, and moreover surface damage of the fixing roller.
In a related art, a cleaning method for the peeling claw in order to cope with such problems has been proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 10-149049 (1998)). In this cleaning method, while the fixing operation is not going on, the heating roller is heated so that the toner adhered to the peeling claw is softened or fused, and by rotating the heating roller in an opposite direction to a direction thereof under the fixing operation, the softened or fused toner is made to transfer from the peeling claw to the heating roller.
However, in the method disclosed by JP-A 10-149049, even when a cleaning operation is carried out, a total amount of softened or fused toner is not always transferred from the peeling claw to the heating roller. In a case where the toner remains on the peeling claw, a large amount of the toner will be accumulated on the peeling claw on a relatively early stage after restart of the fixing operation followed by the cleaning operation, resulting in a need for repetitively executing cleaning operations. Accordingly, there arises a problem that a running efficiency of image formation is made to decrease.